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Full text of "Handbook Of Chemical Engineering - I"

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THE TRANSPORTATION OF LIQUIDS
135
The speed of a pump may be stated as rotative speed in revolutions or cycles per minute, or as piston speed in feet per minute. S = sn/Q, where S = piston speed/ feet per minute; n = revolutions per minute; s = stroke in inches. In direct-acting pumps one revolution means one complete cycle of any reciprocating part. Table I gives the proper speeds of all types of reciprocating pumps.
The mechanical efficiency of a pump is the power utilized in percentage of the power expended. Em = Pump horsepower/Indicated horsepower. Or Em = Pump horsepower-r-Brake horsepower, in the case of a power pump. It accounts for all mechanical losses in transmitting the power from the power end to the pump end. Table VIII gives usual Em for various types of pumps and different lengths of stroke.
TABLE VIII.—MECHANICAL EFFICIENCIES IN PER CENT
Stroke, inches	Direct-acting Pumps				Direct acting water motors	Crank and flywheel pumps
	Plunger pumps and piston pumps	Outside packed pumps up to 300 pounds per sq. inch	Pressure Pumps			
			up to 1,000 pounds per sq. inch	up to 3,000 pounds per sq. inch		
3	50.0	47	45	39	31	
4	55.0	52	50	43	34	
5	60.0	57	54	47	37	
6	65.0	61	58	51	40	
8	70.0	66	63	55	43	
10	75.0	71	67	58	46	
12	77.5	74	70	60	48	80.0
15	80.0	76	72	62	49	82.5
18	82.5	78	74	64	51	85.0
24	85.0	81	77	66	52	87.5
30	85.0	81	77	66	52	87.5
36	87.5	83	79	68	54	90.0
48	90.0	85	81	70	55	92.5
60	90.0	85	81	70	55	92.5
The Volumetric Efficiency of a pump is the capacity in percentage of the displacement. Ev — Q/V. This efficiency gives information as to the losses by leakage. Ev = 1-slip.
The slip, or the water leakage past the plunger and through the valves, in a pump in good condition, running at normal speed, is not over 3 per cent. It is approximately inversely proportional to the speed and at very slow speed may be 100 per cent. This speed, for which the capacity = 0, is of value in testing small pumps; it is expressed by the number of strokes the pump makes with the discharge valve closed and with sufficient pressure on the steam piston to produce the normal water pressure. It is called the lost actions, and should not be more than 1 per cent of the normal speed. The proportion of lost actions to normal speed gives a measure of the slip. The leakage past the plunger may be eliminated in an outside-packed plunger pump by properly packing the stuffing boxes and is very little in a well made plunger and ring